1. Field
This invention relates to an improved process for removing dinitrogen difluoride from a nitrogen trifluoride containing atmosphere.
2. Description of the Prior Art
In the mid 1960's nitrogen trifluoride processes were developed with the nitrogen trifluoride being used primarily for the synthesis of high energy liquid and solid propellants. In recent years the uses of nitrogen trifluoride have changed and now the nitrogen trifluoride, because of its stability under pressure, is being used as a fluorine source for the synthesis of fluorocarbons or as an oxidizer in lasers.
Nitrogen trifluoride can be prepared by several processes, these processes being described in U.S. Pat. Nos. 3,304,248; 3,181,305; 3,356,454; and 3,214,237. The earlier processes used for the preparation of nitrogen trifluoride involved the electrolysis of ammonium bifluoride. In that process ammonia and hydrogen fluoride were introduced into a molten bath of ammonium bifluoride and an electric current was applied to generate nitrogen trifluoride.
Another process for producing nitrogen trifluoride involved the fluorination of ammonia in the presence of a copper catalyst at temperatures from about 0.degree.-50.degree. C.
Another process involved the fluorination of nitrogen and there nitrogen and fluorine were passed through a plasma arc at a temperature of at least 1,000.degree. C.
Basic to each of these processes is the production of other nitrogen fluorides particularly dinitrogen difluoride (difluorodiazine) which must be removed. However, as the art has recognized, difluorodiazine is difficult to remove from a nitrogen trifluoride containing atmosphere. (See U.S. Pat. No. 3,109,711.)
The basic processes employed in removing dinitrogen difluoride from nitrogen trifluoride atmospheres, particularly in the off-gases from the nitrogen trifluoride synthesis reactor have involved adsorption of the dinitrogen difluoride or cryogenic separation and fractionation. U.S. Pat. No. 3,356,454 shows removing difluorodiazine from nitrogen trifluoride containing atmospheres by the use of adsorption equipment filled with an adsorbent material such as silica gel and synthetic zeolite. Temperatures associated with adsorption were about 0.degree. C.
U.S. Pat. No. 3,181,305 discloses a process for separating a tetrafluorohydrazine and nitrous oxide from a nitrogen trifluoride containing atmosphere by cooling the atmosphere to a temperature sufficient to solidify the nitrous oxide, filtering the nitrous oxide from the system, and then cooling to a temperature of -160.degree. C. and separating the liquid nitrogen trifluoride from the immiscible liquid tetrafluorohydrazine.
U.S. Pat. No. 3,214,237 which discloses a process for producing nitrogen trifluoride shows the use of a fractionation procedure for removing dinitrogen difluoride. In this patent the reactor gases are passed through a series of cold traps maintained at -196.degree. C. to produce condensates of nitrogen trifluoride and other nitrogen fluorides such as dinitrogen tetrafluoride and dinitrogen difluoride. These nitrogen fluorides are separated by fractional vaporization or distillation.
The two major disadvantages of separating dinitrogen difluoride from a nitrogen trifluoride containing atmosphere by the prior art techniques are as follows:
(1) The adsorption often is not selective to dinitrogen difluoride, but also can adsorb a significant quantity of nitrogen trifluoride, particularly at low temperatures. This nitrogen trifluoride is lost on regeneration of the adsorbers, and
(2) fractional distillation of the nitrogen trifluoride atmosphere inherently results in concentrating difluorodiazine, and when it is in concentrated form, difficulties in handling can exist as the product exhibits a tendency to decompose.